STS2 Update

Sts2 Users,
Over the last few days, I have done a lot of testing of the STS2 system to
figure out what happened and how to prevent it from happening again. My
conclusions as detailed below is that we have allow users to run with too
high of reflective power and that there a few problems with the matching
network which made things worst. As a consequence there will be new
restrictions on how you can operate the tool.
1. The repaired generator is working fine except for an initial interlock
fault. While my initial impression was the light emission was more uniform
in time, this has not been verified so I am not sure if you will any
changes in etch rates. I now expect that the rates have not changed.
a. If the system has sat unused for a while, you will get an interlock
fault when you start a process. The solution is to clear the error and to
hit the retry bottom at the error page. Next week, I will try to solve
this problem.
2. We loss the generator because of high reflective power, so more
attention has to be paid to keeping the reflected power down. In
particular, in all the recipes we have do a better job in setting up the
plasma striking (tune and load) positions for the matching network, and we
CANNOT allow ETCH FIRST recipes or steps. The reason for the etch first
restriction is that SF6 is an extremely electronegative gas and as such
requires higher voltages to breakdown. The matching network has a hard
time quickly going from matching the breakdown or striking condition to
the inductive coupling condition without generating excessive high
reflective power for SF6.
a. To make sure these new restrictions are followed, for the near term,
I want to approve/test all recipes before they are run. Starting Monday I
will remove all recipes which have not been approved. In the mean time
let me know which one or two recipes you want approved. I will change the
strike settings and test out the recipe for minimum reflective power.
3. Steve Vargo from STS gave me the method for strike positions:
In practice, I usually set the Load position ~5% lower than settled
position and Tune position ~5% higher than settled position. The settled
position is typically where the Load and Tune positions stabilize during
the process and as such would exhibit minimal reflected power. So when
plasma is struck using these start positions the load position increases
and tune position decreases to the settled values.
4. The matching network/control system has a hysteresis problem for both
the load and tune strike positions. In particular for a tune setting of
40% in the recipe during the stabilation step the tune position goes to
42% if it is coming from a higher percentage, such as 60%, and it goes to
the 38% position if it is coming from an initial position of 30%. So far
STS has not given me a way of reducing this hysteresis. This hysteresis
and the use of etch first recipes was the cause for the matching problems
we were seeing before the generator failure. In addition the matching
network was going into an oscillation mode so it was not able to correct
itself. If we restrict ourselves to using etch first and O2 recipes, we
should not see the oscillation mode. In addition, this restriction allows
the matching network to correct for hysteresis problem such that on a
second run of a given recipe the tune and load position will be at the
correct positions when the plasma is begun.
Thanks, Jim
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James (Jim) P. McVittie, Ph.D. Sr. Research Scientist
Paul G. Allen Building Electrical Engineering
Stanford Nanofabrication Facility jmcvittie at stanford.edu
Stanford University Office: (650) 725-3640
Rm. 336X, 330 Serra Mall Lab: (650) 721-6834
Stanford, CA 94305-4075 Fax: (650) 723-4659